1. Field of the Invention
The present invention generally relates to a method for producing a heat exchanger and to the heat exchanger produced by the method. More particularly, the present invention relates to a stack type heat exchanger that is brazed when a header plate disposed at an upper side is supported by jigs and cores and to the method for producing the stack type heat exchanger.
2. Related Art
As illustrated in FIGS. 5 and 6, it is known to stack cores 103 with a plurality of tubes 101 in order to form engine cooling water passages. Furthermore, it is known to assemble a plurality of corrugated fins 102 and connect a pair of header plates 104 to both ends of tubes 101, and also to connect header plates 104 using side plates 105. Cores 103 are hung from an upper header plate 104 in a downward direction. Upper header plate 104 is supported by carbon jigs 106. An assembly including these structure is integrally brazed within a furnace. Each tube 101 is lightly press fitted into each press-fitting hole 107 within the pair of header plates 104. Cores 103 are held between side plates 105 at both sides of the heat exchanger (in some cases, cores 103 are held between side plates 105 and bundled with wires, etc.).
When the assembly is brazed in this state, the brazing material clad on the surface of each tube 101 melts and flows, whereby the width in the direction of the stack of corrugated fins 102 and tubes 101 (lateral direction) is reduced, and at the same time, the reaction force of the corrugated fins 102, side plates 105, etc. is decreased in the high temperature condition associated with brazing. As a result, as illustrated in FIG. 7, a gap t1 is formed between the back of end side 105a of side plate 105 and the back of inner wall 108a of U-shape groove 108 formed on the distal end part of header plate 104.
Furthermore, when the brazing material clad on the surface of each tube 101 melts and flows, the diameter of each tube 101 is reduced, and at the same time, due to the exposure to high temperatures, the press-fitting force of each tube 101 press fits into each press-fitting hole 107 in the header plate 104 is decreased, whereby cores 103 hung from the upper header plate 104 may slide down by due to their own weight. If core 103 slides down from upper header plate 104, side plates 105 also slide down with cores 103. As illustrated in FIG. 7, a gap t2 forms between outermost part 105b of side plate 105 and the back of opposite surface 104a of header plate 104.
If the occurrence of at least one of the above gaps t1 and t2 can be prevented, side plates 105 and header plates 104 may be brazed to each other. If both gaps t1 and t2 occur, defects will be caused such that side plates 105 and header plates 104 are not brazed to each other, thus impairing the strength of the resulting heat exchanger.
Methods have been suggested to prevent cores 103, etc. hung from upper header plate 104 from sliding down due to their own weight. One possible alternative has been to flare the open end of each tube 101 (in other words, to widen the opening and increase the diameter thereof) press fit into each press-fitting hole 107 in header plate 104.
Such a technique has been proposed in Japanese Unexamined Patent Publication No. 59-180295. According to this document, tube flaring pins are inserted into the open ends of each tube 101 press fitted into each press-fitting hole 107 in header plates 104, and the end parts of each tube 101 are flared to an angle within a range from 20.degree. to 30.degree. to improve the connection force between each press-fitting hole 107 and the end parts of each tube 101.
During the brazing process, as core 103 is hung from upper header plate 104 through each tube 101 press fitted into each press-fitting hole 107 in header plate 104, as illustrated in FIG. 8, a force indicated by arrows A is applied to flared part of tube 101. This force (arrow A) is the resultant force made up of a vertical force (arrow B) and a horizontal force (arrow C). On the other hand, each tube 101 is softened during the brazing process due to the high brazing temperature. As a result, the flared angle of the flared part of the tube 101 is narrowed by the vertical force applied to the flared part of tube 101 (arrow B).
If the flared angle of the flared part of the tube 101 is narrowed during the brazing process in this way, the core 103 hung from the upper header plate 104 will slide down due to its weight, whereby, as illustrated in FIG. 7, the gap t2 is formed between the outermost part 105b of the side plate 105 and the opposite surface 104a of the header plate 104.
Another method has been disclosed in U.S. Pat. No. 4,700,469. According to this Patent, the end parts of each tube 101 press fitted into the press-fitting holes 107 in the header plates 104 are flared to an angle of approximately 180.degree.. If the flared angle is set to approximately 180.degree. in this way, any horizontal force will not cause part of the tube 101 to widen, even if the core 103 is hung from the upper header plate 104 through each tube 101. Accordingly, the widened angle of widening part of each tube 101 will not be narrowed during the brazing process, whereby the core 103 hung from the upper header plate 104 will not slide down due to its own weight. As a result, the gap t2 is not formed between the outermost part 105b of the side plate 105 and the opposite surface 104a of the header plate 104.
When the end parts of the tubes 101 are flared to an angle of approximately 180.degree., considerably large loads are applied to the tube 101 and to the header plate 104 supporting the tubes 101. According to the disclosure of U.S. Pat. No. 4,700,469, the end parts of all the tubes 101 are flared to an angle of approximately 180.degree.. Therefore, if the end parts of all the tubes 101 are flared to such an angle, considerably large loads had to be applied to all the tubes 101 and the header plate 104 around the respective tubes 101.
The loads applied to the end parts of the header plate 104 are supported by the side plates 105, which have a sufficiently high strength to support the load, and the tubes 101, which do not have a sufficiently high strength. However, loads applied to the central area of the assembly of cores 103 are supported only by the tubes 101, which is not desirable.
For this reason, if the technique disclosed in the U.S. Pat. No. 4,700,469 is applied, considerably large loads will be applied to the tubes 101 disposed in the central area of the assembly of the cores 103. As a result, defects are caused such as the buckling of the tubes 101 disposed in the central area of the assembly of the cores 103 and warp deformation of the upper header plate 104.